Recently, there has been active development of electronic devices (compound semiconductor devices) in which a gallium nitride (GaN) layer and an AIGaN layer are successively formed on a substrate made up a material such as sapphire, SiC, GaN, or Si, with the GaN layer being used as an electron transit layer.
The bandgap of GaN is 3.4 eV, which is large compared to the 1.1 eV for Si and the 1.4 eV for GaAs. For this reason, high-voltage operation is anticipated for such compound semiconductor devices.
One such compound semiconductor device is a GaN-based high-electron-mobility transistor (HEMT). Hereinafter, such a GaN-based high-electron-mobility transistor will be designated a GaN-HEMT. A HEMT is a field-effect transistor incorporating a high-mobility two-dimensional electron gas (2DEG) induced by a semiconductor heterojunction as a channel.
Using a GaN-HEMT as a switch in an inverter for a power supply simultaneously enables both a reduction in the on-resistance and an improvement in the withstand voltage. Furthermore, reducing the power consumption during standby compared to a Si-based transistor is possible, and improving the operating frequency is also possible.
For this reason, switching loss may be reduced, making it possible to reduce the power consumption of the inverter. In addition, for transistors with equivalent performance, miniaturization compared to a Si-based transistor is possible.
If a GaN-HEMT is operated at a high frequency and high voltage, there occurs a current collapse phenomenon in which the drain current decreases. One conceivable cause of the current collapse phenomenon is that free electrons become trapped in electron trap levels near the drain electrode side of the gate electrode. If electrons become trapped in surface trap levels, a depletion layer is formed in the 2DEG layer, thereby increasing the on-resistance between the source and drain, which may lead to reduced output of the GaN-HEMT. As a countermeasure against the current collapse phenomenon, there exists a technique that provides a field plate electrode having the same potential as the source electrode between the gate and drain, in order to moderate electric field amplification near the gate electrode.
However, when attempting to operate a GaN-HEMT at even higher frequencies and higher voltages, there is a problem in that simply providing a field plate is insufficient to lower the on-resistance between the source and drain.
The following is reference document:    [Document 1] Japanese Laid-Open Patent Publication No. 2006-114795.